The demand for hydrogen as a next generation, clean and renewable energy source has stimulated considerable efforts towards developing sensitive, reliable, and cost effective hydrogen sensors for the fast detection of hydrogen leaks below the lower explosive limit (LEL) of 4.65% by volume ratio of hydrogen to air. Currently, palladium (Pd) or Pd alloys (less than 10 percent metal such as silver or nickel) are commonly used as the sensing material, due to the high sensitivity and selectivity of palladium to hydrogen.
Upon exposure to hydrogen, the Pd and Pd alloy (less than 10 percent metal) absorb hydrogen and form palladium hydride (PdH), a process which can be monitored via the change in reflectance/transmittance or electrical conductivity of the Pd or Pd alloy (less than 10 percent metal). A limiting factor in the fast detection of hydrogen leaks is the intrinsic α to β PdH phase transition that pure Pd or Pd alloy (less than 10 percent metal) undergoes upon exposure to hydrogen concentrations near and below the lower explosive limit (LEL), typically ranging between 0.1% and 2%. For example, response times as long as 3,600 seconds have been observed for optical based palladium hydrogen sensors. Such slow response times at these hydrogen concentrations are characteristic of critical slowing down phenomena that occur at the phase transition of the material. See Kalli et al., K. Kalli, A. Othonos, C. Christofides, J. Appl. Phys. 91, 3829 (2002) and Zhao et al., Anal. Chem. 76, 6321 (2004). In addition, cyclic loading and unloading of hydrogen will also cause mechanical problems like blistering or delamination of the palladium films from substrates leading to poor sensor durability.
An attempt has been made with the addition of silver (Ag) to palladium in the gate metal of metal-insulator-semiconductor gas sensing diodes to improve the performance and change the selectivity of the sensors for a variety of reactions. Diodes with between 10% and 45% silver have been shown to respond to hydrogen gas. For low hydrogen level testing (250-ppm or 0.025% H2), Ag content of over 40% gave poor performance, as well as 15% and 21% alloys. 10% and lower Ag showed faster response than pure Pd. 32% showed a fast initial response with a slow approach to steady state. See, Hughes, et al., “Thin-film palladium and silver alloys and layers for metal-insulator-semiconductor sensors”, J. Appl. Phys. 62 (3), pages 1074-1083, 1 Aug. 1987.
There is a need for further development of hydrogen sensors, and more specifically, to methods for forming palladium alloy thin films such as for use in all-optical based hydrogen sensors with proven reliability and a seconds level response time.